I've posted many times on here about water intrusion and what happens to Filon siding and the structure underneath.
From what I see in the pictures there is serious water intrusion between the outside skin and the inside walls and, if you don't know what to look for inside, it may look fine, but it isn't.
You have a couple issues beside delamination (which is caused my moisture impacting the glue that adheres the Filon skin to the substrate (usually Luan) under it.
The big issue isn't the delamination, the issue is the mold that's growing between the outer skin and the inner walls. That mold is toxic and a health issue and can cause issues with your lungs.
That is a wood framed trailer with fiberglass insulation and the glass is wet and the frame is rotting. I can see it's quite advanced by the fallen out porch lite, the screws have rusted away from moisture and the acid produced by the mold underneath.
If it was me, I would not even bother having it looked at. Any competent shop that does resto work will tell you that it will cost literally thousands of dollars to remediate it.
Best place for that unit is the local landfill. It's unhealthy to use it.
Of course it looks fine inside because you don't know where to look for intrusion or how to look. I'm not trying to be insulting, just the facts.
Anytime you are looking for water intrusion, you start on the roof, by walking on it and feeling for soft spots underfoot. If it's soft, the underlayment is gone. It's rotted away. May never show on the inside however.
What shows on the inside is in the lower corners where the walls meet the floor. If you see any black residue there, it's wet. You look inside cabinets and under the mattress and under the bed where it meets the wall and floor transition and you use a thermal imaging camera or non-contact IR thermometer to gauge the temperature differential between the various interior walls, the wet ones will always be a couple degrees warmer because mold and rot generates heat.
The telling thing is the seller saying he had the roof sealed? If he had done proper seal maintenance in the first place there would not have been an issue.
I live in Michigan as well and any unit kept outside here, year around, with our weather extremes, is a suspect of water intrusion unit.
I would return it and look elsewhere. Even 4 grand is no bargain, you'll spend at least that much remediating it. Probably more.
I only know of one shop in SW Michigan that dies that work and I'm not sure he's taking on any right now.
My 2 cents.